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Monument Future. Siegfried Siegesmund
Читать онлайн.Название Monument Future
Год выпуска 0
isbn 9783963114229
Автор произведения Siegfried Siegesmund
Жанр Документальная литература
Издательство Автор
For a Portuguese limestone (usually named Valverde) several stoneworkers described bowing phenomena. In order to evaluate these observations systematically, eleven limestone varities were selected and analysed: Alpinina (ALP), Ataíja Azul (ATAZ), Ataíja Creme (ATCR), Semi-rijo Codaçal (cut pararellel to the sedimentary lamination, CODFV), Lioz (LIOZ), Moca 4M (cut perpendicular to the sedimentary lamination, MCCT), Semi-rijo Branco Mais(i) (SBM), Semi-rijo Branco Real (SBR), Vidraço Azul Valverde (VAV); Vidraço Portela Azul (VPAZ); Vidraço Portela Creme (VPCR).
The limestone samples used in this study where collected in the Maciço Calcário Estremho (MCE), a Jurassic limestone massif located in the Portuguese Lusitanian Basin, with the exception of one sample (LIOZ) collected in a Cretaceous Unit near Lisbon (Silva, 2017). MCE is one of the world’s leading regions producers of limestones for ornamental purposes. The quarries develop along the hillside, with several benches, can reach depths around 50 m and are grouped in several exploitation sites. Several lithostratigraphic units are exploited and originate different ornamental varieties, from cream laminated calcarenites to grey calciclastic micritic limestones (Carvalho and Lisboa, 2018). The samples selected from MCE show this high variability.
Figure 2: Microstylolites (right in the photo) and non-disturbed matrix (left in the photo) in the sample VAV (crossed polars).
147Besides a petrographic characterization, the hydric expansion caused by moisture exposure was evaluated as well as the bowing potential and thermal expansion measurements were performed.
Material
The detailed petrographic characterization of the Valverde sample highlights some interesting features; the limestone shows a partially recrystallized groundmass, consisting of around 50 % micritic and 20 % of dismicritic components and peloidal microsparit.
Microscopically it is possible to recognize that there are zones (patches) composed of micritic matrix and an other one composed of sparitic cement, giving the rock an inhomogeneous effect similar to bioturbation. All components are smaller than 2 mm composed of bioclasts, pelloids, pseudo-pelloids, benthic foraminifers, echinoderms and several spicules of crinoids. The fabric of the sample varies locally from mud- to grain- supported.
An interesting aspect of this sample is the presence of disturbed areas where parallel microstylolites appear, affecting the groundmass. The microstylolites consist of an opaque material (black) probably accumulations (laminaes) of organic material and clays.
According to Flügel (2004) this limestone belongs to the RMT 14, Bioclastic packstone and wackestone with skeletal grains, various amounts of intraclasts and some ooids (shoal-near). These sediments are common for the open marine, inner ramp zone.
Methods
The hydric expansion was determined for all limestones on cylindrical samples (Ø 15 mm × 100 mm). The length change of those cylinders was measured under water-saturated conditions. The samples were completely immersed in demineralized water for 24 hours, meanwhile the expansion of the samples was measured with a displacement transducer. The resolution of the transducer is 0.1 µm and the accuracy is about 0.5 µm.
Figure 3: Evolution of the moisture expansion during a 24 hours test.
Figure 4: Bowing behavior of the selected limestones under wet and dry cycles.
Figure 5: Bowing effect after heating cycles.
To determine the bowing potential and its directional dependence, slabs of 40 × 10 × 3 cm were exposed to heating cycles (20–80–20 °C per day) simulating in an accelerated procedure the conditions at the building. Additionally, the specimens were 148exposed to thermohydric conditions, as the slabs were moistened on one side and cyclically heated on the reverse slab surface. The slabs were heated for 6 hours before they cool down to ambient temperature and the water was refilled 2 hours before the next heating cycle started. The bowing was measured by means of a measuring bridge with an accuracy of l µm/35 cm. A total of 91 cycles were performed (for details see Koch and Siegesmund 2002).
Figure 6: Progressive increase of residual strain for 20–90–20 °C cycles of the VAV samples. Investigated as a function of the number of heating cycles under dry (3 cycles, red background) and wet (4 cycles, blue background) conditions.
Thermal expansion measurements took place in the temperature range of 20–90 °C using a pushrod dilatometer (for details see Koch and Siegesmund 2004). The specimens had a length of 50 mm and a diameter of 15 mm. The residual strain (RS) was determined within seven heating–cooling cycles: 3 dry cycles 20–90–20 °C and 4 wet cycles 20–90–20 °C, in the Z and XY directions. RS is determined by the ratio between the sample length change after the heating–cooling cycles and the initial length (Shushakova et al. 2013).
Results and Discussion
The values of hydric dilatation are lower than 0.09 mm/m, with excepetion of the Valverde (VAV) samples which reach 0.22–0.26 mm/m.
Usually limestones show low hydric dilatation values, because these rocks have low clay content (Siegesmund and Dürrast, 2014). The dilatation of the VAV sample could be explained by some possible clay content within the dark material observed in microstylolites. Further investigations must be performed to clarify this hypothesis.
The bowing tests show the absence of permanent changes in almost all of the selected limestone. In fact, only the Valverde (VAV) sample is affected by this experiment. In the first 24 wet cycles the bending increases continuosly and reaches the value of 7 mm/m. Followed by dry conditions, the sample shows some recovery down to 6 mm/m within the first cycles. This value remains stable during the next 41 dry cycles. When the wet cycles start again a new increase of the bowing is evident with a maximum expansion around 9 mm/m.
149High values of bowing are frequently observed in marbles (e. g. Siegesmund et al. 2008) but unknown in limestones. Furthermore, as the mineralogy of all selected limestones is similar the bowing of the VAV sample turns strange. The only different factor are the microstylolites, which therefore seem to be the reason for the bowing potential. Since no bending occurs when dry cycles are performed, the water is an important factor. The interaction of heat and water seems to open and offset the microstylitic cracks, avoiding the recontraction and resulting in an irreversible deformation.
The thermal expansion measurements for the VAV sample show some residual strain about 0.25 mm/m after 4 wet cycles, while the performed dry cycles do not exhibit permanent deformation. Additionally, the sample has been measured in two directions, Z and XY, perpendicular and parallel to the microstylolites considering any effect caused by directional dependence. As depicted in Fig. 4, the expansion of both directions is comparable. Consequently no directional dependence of the expansion can be observed.
Residual strain is only observed under wet conditions as well as the bowing behavior is only evident under wet conditions. Despite the values of residual strain are lower than the range of values found in marbles exhibiting bowing (Mennigen et al. 2018), they confirm the bowing behavior of this sample. The areas with parallel microstylolites affecting the groundmass are aleatory distributed in the limestone and each one possibly contributes the total bending of the sample.
These